Foreign substances attached on a substrate such as a semiconductor wafer during a manufacturing process of a semiconductor device result in pattern defects in the device, thereby lowering a manufacturing yield. Accordingly, a process employing plasma, e.g., dry etching and CVD, in a manufacturing process has been widely studied recently. Specifically, a plasma reaction of various gases introduced into a manufacturing apparatus is used to carry out micro processing, e.g., film deposition and etching.
In such a process, deposits (a deposition film) may be attached on an inner wall of the manufacturing apparatus, in addition to an object being micro-processed. Since, for example, molecules of an etching gas may be decomposed or combined with each other in the plasma and etching by-products may be generated during the dry etching process, deposits thereof may be attached on the inner wall of the apparatus during the dry etching process. As the number of wafers treated increases, the deposits get thicker and are partially peeled off to generate dusts, which cause pattern defects in devices. Therefore, such deposits attached on the inner wall in the apparatus must be periodically removed.
If a depo-process is followed by a depoless-process in a same chamber while dry etching a silicon oxide film, an etching rate of the depoless-process becomes unstable, since the depoless-process can be easily influenced by deposits produced in the same chamber during the preceding depo-process. The term “depo-process” used herein denotes a process of using a gas with a smaller F/C (fluorine/carbon) ratio, e.g., C4F8, which leaves a visible amount of polymers including a CF based element such as CF4 attached to the chamber. The term “depoless-process” used herein denotes a process using a gas with a larger F/C ratio, e.g., CF4, which leaves no polymers attached to the chamber.
In order to solve the above problem, a gas reacting with the deposits is conventionally introduced to perform a dry cleaning process. Japanese Patent Laid-Open Publication No. 5-291213 discloses such a dry cleaning process scheme, wherein while a wafer installed on a supporting table is maintained at a temperature below 0° C., a process gas, i.e., a mixed gas of CF4 and O2, is introduced into a chamber so that the deposits may be removed by an activated process gas. Japanese Patent Laid-Open Publication No. 7-78802 discloses another dry cleaning process scheme, wherein a cleaning gas in which H2 gas is added to O2 gas is introduced to perform the dry cleaning process.
The gas used in the dry cleaning process reacts with the deposits of deposition film to be removed to produce compounds of high vapor pressures, so that the deposits are removed by an evaporation and vacuum exhaustion. Since a condition in the chamber just after the dry cleaning process is different from a stable condition in the chamber reached after repeatedly performing the depoless-process, the etching rate in the depoless-process decreases.
To solve the above problem, a seasoning process has been introduced. In the seasoning process, instead of processing the wafer right after the dry cleaning process, a plurality of dummy wafers are transferred into the chamber and a plasma processing is performed on the dummy wafers under exactly same process conditions as those of the actual plasma processing process to stabilize the process conditions in the chamber.
However, the seasoning process requires the dummy wafers for the seasoning process and at least several minutes of processing time. Accordingly, an additional effort and an additional time needed therefore lower an operating rate of a series of plasma processes.